Mr. Foreman: Aerodynamics Part 2

LAST week’s article would inevitably lead to this one. Since as any motorhead is interested in one form of car sport or another, it goes with the territory that you would modify your car in the style of that particular sport. Be it Formula One or the World Rally Championship, most of us hanker to put on stuff that duplicates our hero’s race car. But do we know what effects they have on our car?

Automotive aerodynamics is the science of managing airflow over all parts of the vehicle. The coefficient of drag (Cd) that was discussed last week is managed in many ways. While a low Cd is beneficial for saving fuel or reaching high speeds, it is not always better. Many of the things race teams attached to their cars actually increase Cd appreciably. For instance, car spoilers and wings produce negative lift (which is also known as downforce) and will increase drag (and therefore Cd), but the overall result would be a vehicle’s increased grip on the road, which results in stable cornering.

The trick is to judge the right amount of downforce that is needed for traction and steering, but not so much that excess drag builds up. Conversely, the vehicle’s drag coefficient must be low enough to allow it to effectively accelerate and hold higher speeds on straightaways without creating lift, which can cause instability and forces a driver to decelerate his vehicle in order to handle turns or corners with greater stability. Different parts provide different effects, so pay attention to what you put install on your car.

Front Air Dams

Air dams or front spoilers, are mounted on the front of the car below or as a part of the front bumper. They vary in size but their primary function is to reduce the amount of air flowing under the car. This is because the multitude of parts underneath an average vehicle, which includes the exhaust and suspension systems, as well as drivetrain, create a lot of drag, so less air flowing underneath is a good thing. Furthermore, reducing the amount of air flowing underneath the car forces more of it to travel around the sides and over the bonnet and roof. This serves to create a low-pressure area, which draws the vehicle’s body closer to the ground due to a suction effect.

Subaru Impreza STi front air dam.

Front Splitters

Splitters work in conjunction with air dams and are used to create downforce on the front of a vehicle. They extend forward horizontally, from the bottom of the air dam and create a high-pressure zone in front of the vehicle at high-speeds. As their name imply, splitters separate high-pressure air in front of the vehicle from the high-velocity air flowing underneath the vehicle. Adjustable splitters are used to fine-tune the amount of downforce to maintain an aerodynamic balance under varying conditions. Due to the need for the splitter to be as thin as possible, it is usually constructed from carbon fibre.

Canards

Canards are positioned on the front bumper corners just ahead of wheel well openings. By scooping up air and redirecting it upward, they create additional downforce on the front-end. They are usually found on cars that needs that additional downforce at the front-end for reasons, such as a car with a light front-end or imperfect balance. It is usually made from carbon fibre for the same reason as front splitters. They are also installed on the rear bumper or fender.

Dodge Viper racing canards.

Rear Wings

On a racetrack, it is crucial for a car to create downforce and not lift. Just like an aircraft wing, but upside down, the rear wing creates downforce because of pressure differences above and below the wing. Air passing underneath it is forced to move faster than the air on top as it travels around the curvature of the wing, creating a low-pressure vacuum underneath. The higher pressure on top of the wing then pushes down the vehicle, creating downforce.

Adjustable wings can be angled to increase airflow resistance and create even more downforce.

Race cars typically feature this type of wing to accommodate a range of conditions and racetracks. However, those that have been integrated into a vehicle’s design extremely well will cause little or no drag.

GT3 RS rear wing.

Rear Spoilers

Rear spoilers or ducktails ar e mounted on the car’s upper rear surface or boot and there is no gap between the spoiler and the body similar to the rear wing. Its function and effectiveness is dependent on its size, angle, and shape of the vehicle it’s mounted on, but its main purpose is to smooth the airflow behind the vehicle.

A spoiler can be fitted to either provide downforce or to reduce drag. If a spoiler’s primary function is to provide downforce, it will be tilted at a more vertical angle. As such, high-velocity airflow coming off the roof toward the back of the vehicle will be disrupted, reducing its velocity and creating the air high-pressure that results in downforce. Sometimes it is a necessary item if the shape of the vehicle is as such that it becomes unstable at high speeds due to lift issues.

While downforce spoilers usually increase drag, spoilers can also be fine-tuned to reduce drag. For instance, if the rear edge of a car’s roof drops off sharply, airflow between that point and the trunk lid becomes choppy and turbulent - increasing drag. A properly designed spoiler can, in effect, make the vehicle’s roof slope gentler to reduce drag by preventing airflow separation.

Mercedes C63 AMG spoiler.

Side Skirts

Side Skirts work in union with air dams to minimise the amount of air that works its way underneath the car from the sides. Without them, an air dam’s effect can be negated because the low-pressure area created under the car by the air dam would cause high-pressure air from the sides to be introduced. Just like the air dam, side skirts are more effective when they’re closer to the ground.

Side skirts on a Focus RS.

Rear Diffusers

A rear diffuser is another component that is dependent on the presence of other parts to function effectively. It works with the air dam and side skirts to create a low-pressure pocket underneath the car. It redirects underbody airflow by providing widening channels to expand and decelerate the flow of air. Once it is been slowed down, that air will now be moving at the same speed as the airstream that travelled over and around the vehicle. This minimises turbulence will help the other components work better.

Mustang GT rear diffuser.

We may not be able to afford a wind tunnel test, but with common sense and much (road) testing, it is possible that we can make our car perform better. Just bear in mind that fuel economy (and top speed) goes right out the window once you start adding downforce items. Still, we can’t help ourselves, right?